A sperm-sorting machine could soon allow fertility clinics to filter out sperm that have a type of DNA damage associated with infertility and a heightened risk of childhood cancers in offspring.

Mechanised sorting should be particularly useful when a would-be father is older, or is a heavy smoker or has been exposed to pollution in the workplace - all factors that increase this type of DNA damage.

When attempting IVF with sperm from men with fertility problems, clinics usually centrifuge the semen to increase the concentration of the densest sperm cells, which tend to be the healthier ones. For certain fertility treatments, technicians also try to pick the healthiest-looking sperm, for example, those with regular, ovoid heads.

But the centrifuge alone takes at least 45 minutes to sort sperm, which is a long time when processing hundreds of samples in a busy clinic, and neither technique can spot sperm that are free of DNA damage. Enter the new sperm sorter, developed in Australia by John Aitken and Chris Ainsworth at the University of Newcastle in New South Wales, alongside commercial partner Life Therapeutics of Sydney.

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20-day voyage

The sorter is based on the principle that sperm with the most negatively charged membranes have the least DNA damage. Aitken is not sure why this is the case, but he speculates that it may simply be that these sperm are more likely to have matured normally.

Maturing sperm make a 20-day voyage through the epididymis, the 6-metre long, tightly coiled duct between the testes and vas deferens. As they progress, they acquire more and more negatively charged CD52 proteins on their membranes.

The sorter, which is smaller than a shoebox, comprises two chambers separated by a polycarbonate filter shot through with holes that are 5 micrometres in diameter. Sperm are the smallest of human cells and the holes are large enough to let them through while blocking other bodies such as white blood cells, which often contaminate semen and can damage sperm.
Visual selection

Semen is injected into the first chamber and a voltage is applied across the filter for up to 5 minutes to move the more negatively charged sperm into the second chamber.

In preliminary tests, using semen from medical students, the 20% of sperm that made it into the second chamber had only half as much DNA damage as the sperm left behind. Other tests suggest that the sorter is equally efficient at pulling sperm with less DNA damage from the semen of men with fertility problems. Interestingly, the sperm that were selected also looked healthier, with the same physical characteristics sought by technicians making a visual selection.

"It is so simple. I've never seen anything like it before. You turn it on, the sperm move across and there you go," says Moira O'Bryan of the Monash Institute of Reproduction and Development in Melbourne, Australia. "Only time will tell, but it might take some of the subjective nature out of picking good sperm."

The sorter's ability to select "good" sperm will be tested in two clinical trials of women undergoing IVF at a fertility clinic in Australia later this year.